Ideally, the function of the lubrication system of an internal combustion engine is to supply clean oil cooled to the proper viscosity to the critical points in the engine, where the motion of the parts produces hydro-dynamic oil films to separate and support the various rubbing surfaces.
Since the lubricating oil flows to all parts of the engine, it is important that it does not carry abrasive or corrosive material with it. Such material may come from the combustion of the fuel, from dirt in the inducted air, or from parts of the engine itself. It is common practice to filter oil or part of the oil as it flows through the system to reduce wear.
In some engines a small fraction of the oil leaving the pump is continually bypassed through a filter and returned to the crank case. The rest of the oil is directed to the bearings.
However, since considerable time is required for all the oil to be filtered by this method, most automobile engines pump all the oil through a full flow filter placed in line between the pump and the bearings.
Ideally, such a full flow filter would filter out of the oil all the abrasive materials and particles in the oil, even the microscopic metal particles which are abraded from the moving surfaces of the engine in the normal wearing process.
However, for a filter to remove all of this material, the filter would have to be designed and be constructed of a material suitable to remove these microscopic particles, and such a filter would severely impede the flow of oil to a degree where insufficient oil would flow to the bearing, and also would require excessively high pressures to force the oil through this fine material.
Hence, presently designed filters are such that they remove all the larger abrasive particles, and their filtering element is also designed to allow the additives in the cell to circulate with the oil and not be filtered therefrom, these additives including the viscosity-index improvers, the antioxidants, the anti-wear and friction reducing additives such as the alkaline-earth phenates and the dispersants which additives have effect to ensure that foreign particles are kept in suspension in the lubricating oil and are not to form sludges and lacquers on the various component parts.
Also in this connection, various wear reducing additives such as graphite and molybdenum disulfide are added to the oils in order to assist in the lubrication of the engine.
The oil filter ideally should remove all abrasive particles, no matter how microscopic, from the oil but should also allow the desired additives in the oil to circulate and clearly remain in the oil. However, the filters are designed to allow the oil and its additives to freely circulate, and thus do not effectively remove the minute abrasive particles which circulate with the oil as a result of the normal wearing process which occurs in the engine.